Yield Stress of Concentrated Zirconia Suspensions: Correlation with Particle Interactions

Abstract

The presence of a sufficient concentration of solid particles in a solution gives rise to a large increase in its viscosity and, more importantly, to significant deviations with respect to its original Newtonian behavior. Different rheological techniques are available to characterize such deviations, but the simplest one, obtention of steady-state rheograms, is already extremely useful with that purpose. In this work, this technique is applied to suspensions of zirconia particles, both synthesized with spherical geometry and commercial. The σ(shear stress)−γ(shear rate) curves show that the suspensions are nonideal plastic, thus exhibiting a finite yield stress, σ0, and a shear-thinning flow. It is through σ0 that a connection can be established between steady-state rheological behavior and interaction energy between particles, since σ0 can be estimated as the maximum attractive force between particles multiplied by the number of bonds per unit area between a given particle and its neighbors. Having an experimental determination of σ0, the verification of its relation with the attractive forces requires estimation of the potential energy of interaction between any pair of particles. Two approaches will be considered: one is the classical DLVO model, in which the potential energy, V, is the sum of the van der Waals (VLW) and electrostatic (VEL) contributions. The second approach is the so-called extended DLVO theory, in which the acid–base interaction VAB (related to the hydrophilic repulsion or hydrophobic attraction between the particles) is considered in addition to VLW and VEL. The three contributions can be calculated as a function of the interparticle distance if the particle–solution interface is characterized from both the electric and the thermodynamic points of view. The former is carried out by means of electrophoretic mobility measurements and the latter by contact angle determinations for three probe liquids on zirconia powder layers. Comparison between measured and calculated σ0 values was carried out for suspensions of spherical, monodisperse ZrO2 particles, with volume fraction of solids, φ, ranging between 4.6 and 21.7%, in 10−3 M NaCl solutions. In the case of commercial particles, the effects of both NaCl concentration (10−5 to 10−1 M) and volume fraction (3.5 to 21%) were investigated. It is found that the classical DLVO theory cannot be used to predict the yield stress when [NaCl]=10−5 M, since the high zeta potentials and thick double layers never yield ∂ V/∂ R>0 (the interaction is repulsive for all distances) in such a case. A similar problem was encountered in 10−1 M solutions, but now because V is always attractive, and no maximum force can be found. On the contrary, the extended DLVO model always yield physically reasonable σ0 values (coincident with those deduced from the classical approach when calculation is possible in the latter case). The comparison with experimental data shows that theory clearly underestimates σ0 by one order of magnitude or even more. The possible role of particle aggregation in this underestimation is discussed in terms of the scaling behavior of σ0 as a function of φ.